Light olefins, defined herein as ethylene, propylene, butylene and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, light olefins are produced by cracking petroleum feeds. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies based on alternative feeds have increased.
An important type of alternate feed for the production of light olefins is oxygenate, such as, for example, alcohols, particularly methanol, ethanol, n-propanol, and iso-propanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
Oxygenate-containing feeds may contain impurities which are deleterious to the catalysts employed in oxygenate to olefin conversion processes. Such impurities comprise non-volatile materials which have negligible vapor pressure at the conditions necessary to prepare feed for the oxygenates to olefin conversion process. Typically, these conditions include temperatures ranging from about 32° to about 500° F., and pressures ranging from about 20 psia to about 150 psia, say from about 50 to about 95 psia. A more detailed description of non-volatile materials is provided below with the description of the present invention.
The conversion of oxygenates to olefins (OTO) takes place at a relatively high temperature, generally higher than about 250° C., preferably higher than about 300° C. Because the conversion reaction is exothermic, the effluent typically has a higher temperature than the initial temperature in the reactor. Many methods and/or process schemes have been proposed to manage the heat of reaction generated from the oxygenate conversion reaction inside of the reactor in order to avoid temperature surges and hot spots, and thereby to reduce the rate of catalyst deactivation and reduce the production of undesirable products, such as methane, ethane, carbon monoxide and carbonaceous deposits or coke. These solutions involve heating the feed to proper temperature and pressure. However, this can lead to problems with the feed such as fouling. Although heavy hydrocarbons, metals and other non-volatile materials are not normally found in freshly produced oxygenated hydrocarbons, non-volatiles can be introduced during storage and handling of oxygenates, as well as during recycling of oxygenate streams to a reactor. Because of the relatively long residence times of catalyst in an oxygenates to olefins conversion reactor, even small amounts of non-volatile impurities/contaminants such as metals, salts and heavy hydrocarbons in the feed can accumulate on and thus poison the reactor's catalyst. These poisons interfere with the catalyst's function, reducing the efficiency of the catalyst and increasing the overall production costs. Given that non-volatiles in the feed at levels as low as one wppm can accumulate to 12000 wppm on the catalyst inventory, a compelling interest exists to provide feeds of extremely reduced non-volatiles content. Other processes have been taught that attempt to reduce the amount of catalyst poisons in the OTO reactor feed.
One of the processes that utilizes control of the OTO effluent temperature can be found in U.S. patent application Ser. No. 10/020,732, filed Oct. 30, 2001, entitled “Heat Recovery in an Olefin Production Process.” In this application, which is incorporated herein by reference, a process for removing heat from an effluent stream while maintaining a temperature of the gas phase above the dew point is taught. By following this process, solid particles and some other contaminants may be separated from oxygenates. By removing these contaminants, the catalysts in the OTO reactor will perform better and last longer, thereby improving the reactor's efficiency and reducing production costs. Unfortunately, this process does not remove all catalyst poisons and further creates difficulties in maintaining the effluent at a proper temperature and pressure.
Improving the heating of an oxygenate-containing feed is described in U.S. Pat. No. 6,121,504, which is incorporated herein by reference. This patent teaches a series of heat exchangers to heat a feed using heat from the products of an OTO reactor. While this process efficiently heats the effluent, contaminants may still be present at the point where the effluent is fed into the OTO reactor, leading to the aforementioned problems downstream. Contaminants can be introduced not only from vapor feed streams, but from any liquid feed streams that may be used for reactor temperature control as well, e.g., where a reactor outlet temperature below that temperature obtained by equilibrium reactor heat balance for all vapor feed is desired. Accordingly, it is desirable that any liquid feed streams to the OTO reactor be very low in contaminant content, such as streams produced by condensing a clean vapor oxygenate stream, e.g., vaporized feed or recycle oxygenates streams that have been revaporized to remove non-volatiles.
It should thus be appreciated that a delicate balance exists between maintaining an oxygenate-containing feed for an OTO reactor at proper temperatures, while at the same time reducing or eliminating contaminants of such feed. Accordingly, it would be desirable to provide a process that effects substantial removal of contaminants that may poison an OTO reactor catalyst, while at the same time maintaining an effluent at a proper temperature and pressure.